Super Insulation; Thicker Walls for Thicker Insulation

by Roy Baller
(New York)

Greatly increasing the thickness of house insulation—a technique called super insulating is one of the best ways to slash heating bills. In some newly built super-insulated houses with wall insulation values of R-30 and higher and roof insulation values of R-50 and higher—the savings can be as much as 70 per cent com-pared with the cost of heating a similar house with conventional insulation.

Super insulating an older house can yield savings approaching these by stopping drafts and reinforcing inadequate insulation while improving comfort. Many features found in new super insulated houses thermal blocks between structural parts to interrupt the flow of heat, for example, and special roof sheathing are impractical for existing homes. But other super insulating techniques are adaptable to existing houses. The primary technique is to bolster existing insulation with thick extra layers.

It is simple, for instance, to lay extra fiberglass batting in an attic, adding an R value of about 3.33 per inch of thickness. It is also possible to superinsulate exterior walls. Before extra insulation can be added, the exterior walls must be thickened to accommodate it. And thickening the walls involves major construction: A wall frame must be built, then covered over with an attractive surfacing material. the new insulation is installed. In almost every case this frame, which can be built of 2-by-4s or 2-by-3s, is best added to the inside of an existing wall. It can rest on the existing floor and can be covered with inexpensive wallboard.

Thickening walls on the outside requires a heavy ledger board bolted to the studs to support the new frame, plus expensive exterior siding. However, if your house is due for new siding anyway, superinsulating from the outside may prove feasible. The major drawback to superinsulating a wall inside is that living space must be sacrificed. A new wall cavity providing room for 6 inches of new insulation will use up 5 square feet of space for every 10 feet of wall thickened; fully superinsulating a two-story, 20-by-40-foot house means the loss of 120 square feet.

Of course, space can be saved by superinsulating only some of the walls. The south wall warmed by the sun for most of the day needs superinsulation only in the harshest climates. Or limit your superinsulation to the'north wall, or to the north wall and the west or east wall, whichever faces the direction of prevailing winds in your locale. The major physical obstacles to super-insulating walls are windows, doors, electrical boxes and heating outlets. For windows and doors, use 2-by-2 furring strips to extend the jambs back into the rooms; the recesses inside the extended jambs can be covered with wallboard. Electrical cables connected to boxes, forced-air ducts and radiator pipes must be brought into the new room perimeter. The vapor barrier of a superinsulated wall also requires special consideration:

It must be better than its counterpart in an ordinary wall. Because less air seeps through a superinsulated wall, less moisture is carried out of the wall cavity. Instead, any moisture that gets inside the wall can condense, matting and ruining the new insulation. The best defense is a continuous sheet of 6-mil polyethylene sheeting, spread over the new insulation. The sheeting will bolster the performance of the standard foil vapor barrier that comes with the new insulation. If there is an existing vapor barrier, it must be perforated so that moisture can pass through it. Otherwise, even a tiny amount of moisture that penetrated the wall would be permanently trapped be-tween two virtually impervious barriers.

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